Hello experts,

I'm making an HMI for an old S7-315 PLC with WinCC Advanced. Suddenly some of my SetBitInTag's did not update the integer tag in the PLC. I use SetBitInTag to activate actions i the PLC. The PLC sets them back to "false" when the action is accomplished. In the WinCC HMI I also have the same bits triggering alarms, and this works fine every time.

After I discovered the problem I tested with some bits in the same tag. When I use SetBitInTag on bit 15 and 13 the integer are updated in the PLC, but when i set bit 0 and 9, they are not updated. I have used the same bits to change color on the respective buttons. Works fine. For testing I have set the acquisition cycle to 10 s. WinCC responds as expected, but the SetBitInTag only updates some bits in the PLC tag. The PLC tag is direct MW.

I have searched the forum for solutions. I see that others have had similar problems, but of more advanced character. Since my problem very simple, I hoped someone may had a simple solution.

The WinCC PC in question is not allowed to have access to internet, so I can't use TIA UPDATER (I operate it via a vpn-connection).

Best regards,

Kåre

Hello AutoGen_5145465,

Are these bits that are not updated being used somewhere in the PLC program?

When using SetBitInTag, it is forbidden to write values from different sources (from the HMI and the PLC). I recommend checking the cross-references to see where the tags are being used.

JBS98

Hello AutoGen_5145465,

Are these bits that are not updated being used somewhere in the PLC program?

When using SetBitInTag, it is forbidden to write values from different sources (from the HMI and the PLC). I recommend checking the cross-references to see where the tags are being used.

 Thank you for answering my question. 

The  bit_n in a tag is set to TRUE by a button in HMI with the instruction SetBitInTag. The tag is integer or word.I used this construction:

- IF bit_n THEN DoSomething; bit_n:=FALSE; END_IF;

Some further experiments lead to the conclusion that the construction above never will work. (According to my logic sense it should.)

What I found: If the bit is set to zero/false in the same program cycle as it is read, then nothing happens at all. I believe it has something to do with how the system handles concurrent operations.

I had to DoSomething only once, so the solution became:

- IF bit_n AND NOT block_n THEN DoSomething; block_n:=TRUE; ELSE block_n:=FALSE; END_IF;

I then added a timer to reset the whole integer/word to zero after 2 seconds.

Regards,

Kåre

I suggest that your PLC should not be writing to the same tag as the HMI at all. I would suggest assigning a separate tag using bit_n, such as this
#bit_n            #bit_x
|   |----------------(s)

That way you can avoid the same tag will not be written from 2 sources, and HMI can still read #bit_x if needed to know if your doSomething() process is complete.